YMMV - yes, your mileage may vary...
what it means is that if you're a lucky sonoabiotch, you'll get the deal to work for you.
Some of the ymmv deals refer to clearance items, which aren't priced the same in all stores - the price will depend on the store. (for instance, some staples clearance the xmas cookies and mark them down from $6.95 per tin to $1 to get rid of them... my local staples clearanced the 6.95 tins down to $6 - not a deal at all.)
Other ymmv depends on local store pricing - you'd think that stores like target and walmart have prices set at the national level, unfortunately, prices are set regionally or locally, except for sale items, which again may be regional or local sales.
And, the worst ymmv deals of all require you to (unethically, I may add) get management to make a mistake by not knowing their pricematching policies like the back of their hands..
Occasionally, some fortunate soul is able to get a store to price match an item, THEN get a rebate on that item. Often, the pricematch policy is to match the after rebate price.
i.e. Store A has an item for $100 with a $30 store (not manufacturer) rebate.
Store B has the same item for $80.
Some people expect store A to pricematch the $80, THEN give a $30 rebate. - generally this is against their pricematch policy and any manager should shoot down the deal.
However, in the case above, the best deal is to get store B to pricematch the $70 after rebate price at store A - saving 10 dollars and avoiding the tax on the difference between 70 and 100.